In electrophotography, an image is produced by adhering colored powder material, such as toner, etc., to the image on a recording medium. As a result, however, a mirror gloss (JIS Z 8741, hereinafter simply referred to as gloss) of the image changes sometimes. That is, the gloss generally increases as the amount of toner attracted onto the image increases. In addition, the amount of toner attracted onto the image is not uniform but fluctuates by section. Specifically, the gloss increases at a solid image section and decreases at a halftone dot section. As a result, unintended uneven gloss appears, thereby degrading the quality of the image.
To solve such a problem, a clear, colorless toner (i.e., toner that does not affect color) may be utilized, as described, for example, in Japanese Patent No. 3,146,367 (JP-3146367-B) and Japanese Patent Application Laid Open No. H9-200551 (JP-H09-200551-A). Specifically, a prescribed amount of clear toner is adhered onto an image so that the total thickness of the toner layer, that is, the total amount of both yellow, magenta, cyan, and black (herein after simply referred to as YMCK color) toner and clear toner, is constant throughout the image. Accordingly, the gloss of the image can also be kept constant regardless of a reproduction color.
In Japanese Patent JP-3146367-B, it is assumed that image information at each of Y, M, C, K pixels has a prescribed analog value or a multilevel digital value greater than 256. Further, an amount of toner attracted to each of Y, M, C, K pixels on a sheet is assumed to have a prescribed analog value (i.e., a toner weight per unit area (e.g. mg/cm2)). Similarly, image information at each clear pixel and the amount of clear toner attracted onto a sheet are each treated as analog quantities. Accordingly, the total amount of attracted Y, M, C, K color and clear toner can be kept constant without great difficulty.
However, a conventional electrophotographic apparatus or a commercial printer cannot directly reproduce a halftone image using halftone toner in accordance with halftone image information expressed as an analog value as described above. Therefore, a dither method is employed to apply binary processing (i.e., either adhering toner or not) and form a halftone dot per pixel, thereby virtually expressing the halftone.
The halftone dots have characteristics such as a number of screen lines, an angle of a screen, a shape of a screen, etc. The screen shape is either a dot or a line and the like, and this discussion assumes the former. In general, a fine halftone dot image is obtained when the number of screen lines increases (i.e., a thin line). However, if the screen line is too thin, uneven density or a similar problem occurs, thereby degrading image quality. Accordingly, the number of screen lines is set to range from about 140 lines/inch to about 200 lines/inch in accordance with the capabilities of the printing (screen) system. Further, to avoid creating a moiré image the screen angles of the component colors Y, M, C, and K are different for each component color such that an indigo or cyan plate (C) is 15 degrees, a magenta plate (M) is 75 degrees, a yellow plate (Y) is 0 degrees, and a black plate (K) is 45 degrees.
In an actual printing system, binary halftone dots of the component colors Y, M, C, and K are superimposed virtually at random in a halftone image region on the recording medium. However, when the total amount of toner is kept constant using the clear toner, a clear toner image signal sometimes becomes irregular or intermittent, and accordingly does not form fine, stable dots like the stable halftone dots of the component colors Y, M, C, and K. Consequently, the surface of the image cannot be completely smoothed, thereby causing uneven gloss and degrading image quality.
Further, when a clear toner is attracted onto a concavity in the image surface, the surface is flattened and a high degree of gloss can be achieved. At the same time, however, convexoconcavity, when the clear toner is attracted onto a convexity on the image surface, that convexoconcavity increases and accordingly the gloss decreases. Hence, the glossiness can be controlled if the clear toner is selectively attracted.